So far, only the asymmetric time division duplex (time division duplex, TDD) spectrums and symmetric frequency division duplex (frequency division duplex, FDD) spectrums are available. In a TDD system, uplink and downlink signals are sent on the same carrier on a time division basis, and the uplink and downlink channels are reciprocal. In an FDD system, the bidirectional communication of receiving and transmitting is implemented on two separate symmetric frequency channels; a guard band is used to separate the receiving and transmitting channels; an uplink carrier only sends uplink signals and a downlink carrier only sends downlink signals.
FIG. 1 shows an existing FDD frame structure in a 3GPP LTE (long term evolution) network. A 10 ms radio frame includes 10 radio subframes; each radio subframe is a 1 ms downlink subframe, and each downlink subframe includes two 0.5 ms timeslots. Because a sounding reference symbol (sounding reference symbol, SRS) of the FDD system can only be sent on an uplink carrier, an evolved NodeB (eNB) cannot obtain the channel information of a downlink carrier according to the conventional measurement of the SRS of the uplink band.
Advanced multi-antenna technologies, including downlink coordinated multi-point transmission (coordinated multi-point transmission, CoMP) and beam forming (beam forming, BF), require the eNB to obtain complete channel information of the downlink carrier, but the existing FDD system can only feed back quantized and encoded channel information of the downlink carrier through the uplink control channel of the uplink carrier. Because the eNB cannot obtain complete channel information of the downlink carrier and can only obtain channel information of the downlink carrier on the basis of uplink feedback information, the system performance is deteriorated greatly.
One solution provided in the prior art is as follows: The downlink carrier carries an uplink SRS to achieve the reciprocity of uplink and downlink channels of the downlink carrier, so that the downlink channel information can be obtained. The eNB obtains the downlink channel information by measuring the uplink SRS sent on the downlink carrier, and may obtain complete and accurate downlink channel information easily. Therefore, the advanced multi-antenna technologies such as CoMP and BF can be used effectively, uplink band overheads can be reduced significantly, and the imbalance of the uplink and downlink of the existing FDD system is improved.
During the implementation of the present invention, the inventor finds the following problems in the prior art:
The FDD downlink carrier carries an uplink SRS. Therefore, the FDD downlink carrier may carry two frame structures, one frame structure is the existing frame structure in the LTE network shown in FIG. 1, and the other frame structure is a new frame structure carrying uplink timeslots/subframes. After the new frame structure is introduced, LTE-advanced (LTE-advanced, LTE-A) users need to identify the new frame structure when accessing the network initially, and then can perform normal communication; the original LTE users should be unaware of the change of the frame structure, and finish the initial access process in the LTE-A network normally, so as to ensure backward compatibility of the LTE-A network.